1. Field of the Invention
The present invention relates to testing circuitry, and more particularly to circuitry used in testing the proper operation of a digital display.
2. Description of the Prior Art
A wide variety of electronic equipment incorporates the use of displays to indicate status and instructions to an operator or user of the equipment. Often these displays are digital displays, comprised of segments that are individually activated to represent digits or rudimentary alphabetic characters. The most common configuration of characters for these displays requires the use of seven segments.
Displays are generally available in three categories: planar gas discharge (PCG), light emitting diode (LED), and vacuum fluorescent display (VFD). In operation, all of these types of digital displays are most often time multiplexed. That is, when the display appears on, different characters are actually being activated or energized but not simultaneously. Only one character at a time is energized. As each character is enabled, some combination of segments is turned on, resulting in an energized digit that appears in the enabled character location. Each character is enabled in this way, in sequence, at a rate fast enough to ensure that human eyes perceive display characters to be on constantly, without appearing to flash or flicker.
The operation of digital displays can be appreciated with reference to the description thereof in U.S. Pat. No. 4,224,506, issued to Coppola et al and assigned to the assignee of the present invention.
Manufacturers have discovered that portions of these digital displays may fail under certain circumstances, such as when a filament burns out, when internal wiring breaks or short circuits, or when a tube cracks, destroying the vacuum. When a failure occurs or is about to occur, it may be important that the operator of a device incorporating a digital display be aware of such a failure. Without this knowledge, the operator may obtain undetected erroneous results. For example, if the center segment of a character "8" fails, the character "0" is displayed. The operator would thus be unaware of the failure resulting in this undetected erroneous display.
The need for determining whether a digital display is operating properly has long been recognized. U.S. Pat. No. 3,742,356, issued to Brock, for example, discloses a method and apparatus for testing LED displays. The apparatus disclosed therein, however, is for specific use with LED's, in which sockets for receiving integrated circuit packages are sequenced at a predetermined rate with the number of sockets being sequenced equal to the reciprocal of a duty cycle at which the diodes are being tested. The magnitude of the voltage being supplied by the source varies as a function of the duty cycle and the average current rating of an LED.
An operator using the Brock apparatus is not made aware of a display failure unless he visually inspects each segment of each character in the display for markedly less brightness or a total lack of display in one of the segments. Moreover, the testing apparatus disclosed in the Brock reference is not intended to be used while the display is in operative relationship to the device with which it is to be used. Rather, the display must be disconnected from its associated device for testing purposes, making the testing of installed displays extremely inconvenient, if not impossible.